To gauge the pathophysiologic relevance of low arginine to myeloma growth independently of immunoevasion, we xenotransplanted human myeloma cells subcutaneously into T cell-deficient Rag2–/–γc–/– recipient mice and treated palpable tumor-bearing mice with the clinical-grade arginase inhibitor CB1158. The gene discussed is GC; the disease is plasma cell myeloma.